linux-kernel - Re: [PATCH] doc: memory-barriers.txt: Add barrier() to provide ordering

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Message-ID: <20140813225017.GD4752@linux.vnet.ibm.com>
Date:	Wed, 13 Aug 2014 15:50:17 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	David Howells <dhowells@...hat.com>,
	Masanari Iida <standby24x7@...il.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] doc: memory-barriers.txt: Add barrier() to provide
 ordering

On Mon, Aug 11, 2014 at 06:51:40PM -0400, Pranith Kumar wrote:
> In the provably false case, add barrier() in both the legs of the if() condition
> to provide ordering.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> ---
>  Documentation/memory-barriers.txt | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 41a6c9c..0f2903f 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -689,9 +689,11 @@ should do something like the following:
>  	q = ACCESS_ONCE(a);
>  	BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
>  	if (q % MAX) {
> +		barrier();
>  		ACCESS_ONCE(b) = p;
>  		do_something();
>  	} else {
> +		barrier();
>  		ACCESS_ONCE(b) = p;
>  		do_something_else();
>  	}

This section had to be rewritten due to -O3.

							Thanx, Paul

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